Non-chemical thermally printable film

ABSTRACT

A two-layer mono-axially oriented film includes a first layer of an opaque beta-nucleated microvoided propylene-based polymer; and a second layer containing a dark pigment that is adapted for use in a thermal printer in which the thermal print-head contacts the exposed surface of the first layer. The dark pigment of the second layer pigment has a color contrasting with the color of the first layer and can contain a carbon black. The first layer includes microvoids and may be made transparent upon the application of heat by collapsing the voids of the first layer or upon the application of ultra-sonic energy.

REFERENCE TO RELATED APPLICATIONS

This present application is a divisional of U.S. patent application Ser.No. 14/555,096, filed Nov. 26, 2014, which is a continuation of U.S.patent application Ser. No. 13/857,374, filed Apr. 5, 2013, now grantedU.S. Pat. No. 8,968,863, issued Mar. 3, 2015, which claims priority ofU.S. Provisional Application No. 61/621,173, filed Apr. 6, 2012. Theentire contents of which are incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

This invention relates to an oriented propylene-based film whichexhibits an appearance change when subjected to heat that does notinvolve any chemical change. This film is well-suited as a label filmthat can be activated by a thermal print-head to provide custominformation. This invention provides a visual change based on anonreversible physical change in the film, obviating the need forchemical reactive coatings or at least minimizing the amount of coatingused. Another feature of this invention is that it is not limited to ablack-and-white color contrast for images or graphics. Activation canalso be accomplished by ultrasonic means.

This film is suitable as an “on-demand” label film for custom labeling,bar-code printing, cash register receipts, for consumer, industrial, andretail applications. It may be part of a laminate structure involvingmultiple films, or as a single laminate web for these applications.

BACKGROUND OF THE INVENTION

For creating custom information on labels, such as data or bar codes, oreven cash receipts, a common practice has been to coat either paper orplastic substrates with a thermal reactive ink that reacts to the heatfrom a thermal print-head to create a visible contrast on the substratematerial.

With the increasing scrutiny of Bisphenol-A—which is a common ingredientor component of such thermal ink coatings—the current thermal printablepapers are under increasing pressure to change the composition of thechemicals that create this color contrast. The current approach withthermally reactive coated papers or labels leaves unreacted chemicals onthe substrate surface which can expose the public consumer to doses ofchemicals that have been shown to have detrimental side effectsparticularly in children but also in adults, even at low exposuredosages.

Another problem with thermal-change inks is that the image is notstable. It is common knowledge that with time, faxes, receipts, andlabels that are printed with these thermal inks often either fade or gocompletely dark with aging. This is particularly true if exposed tohigher temperatures or sunlight.

U.S. Pat. No. 4,004,065 describes a heat sensitive recording membercomposed of a support and a heal sensitive layer overlying the support.The heat sensitive layer contains an iron salt of a higher fatty acidand gallic acid as color forming components, a stilbene seriesfluorescent dye as an unusual color forming inhibitor, and hydroxypropylcellulose and hydroxypropyl methyl cellulose.

U.S. Pat. No. 4,602,265 describes a heal sensitive color-producingmultilayer coating including a first coating layer formed from a basepolymeric coating composition comprising a solution of film-formingpolymer, a source of polyvalent metallic ions, and at least one fattyadd or derivative thereof; a second coating layer, on the first coatinglayer, formed from a sensitizing coating composition comprising asolution of organic film-forming polymer, at !east one fatty acid orderivative thereof, and a reducing agent selected from catechol,pyrogallol, hydroquinone, diphenyl carbazides, thereof; and a thirdcoaling layer, on the second coating layer formed from a base polymericcoaling composition as defined above.

U.S. Pat. No. 5,863,859 describes a heat-sensitive recording materialsuited for use in direct thermal imaging, wherein the recording materialincludes: (i) a layer (1) uniformly distributed in a film-formingwater-insoluble resin binder a substantially light-insensitive organicmetal salt, preferably a silver salt, and (ii) a layer (2) in directcontact with said layer (1) or in thermal working relationship therewiththrough the intermediary of a spacer layer (3), characterized in thatthe layer (2) contains, uniformly distributed in a film-formingwater-soluble hydrophilic binder at least one organic reducing agentthat is capable of diffusing out of said layer (2) Into said layer (1)on heating said recording material, and is coated from an aqueoussolution.

U.S. Patent Publication No. 2008/0233290 A1 describes a method ofpreparing a thermally printable sheet which includes providing asubstrate including a base sheet having at least one surface coated witha layer containing a pigment in solid porous particulate form, and,using a printer, printing onto the coated surface of the substrate, athermal ink which includes a color former, a color developer, which canbe bisphenol A., and a sensitizer, characterized in that the sensitizerincludes dimethyl terephthalate and the ink also includes at least onepigment. This publication also discloses a thermally printable sheetsuitable for use In such a method.

U.S. Patent Publication No 2009/0031921 A1 describes a thermal ink whichincludes a color former, a color developer and a sensitizer, in whichthe color former can be 3-dibutylamino-6-methyl-7-an.anilinofluoran; thecolor developer can be bisphenol A; and the sensitizer can be dimethylterephthalate, and the ink also comprises at least one pigment This inkmay be used in thermal papers to reduce unwanted discoloration duringstorage.

U.S. Pat. No. 6,104,422 describes a sublimation thermal image transferrecording method for thermally forming images on an image-receivingsheet prepared by forming a dye-receiving layer on a substrate. Thedye-receiving layer contains a subliminal dye-containing ink, such as C.I. Disperse, Yellow, Red, Blue; and a binder resin such as polyvinylbutyral or styrene-maleic acid copolymer.

U.S. Pat. No. 4,415,615 describes a cellular pressure-sensitive adhesivemembrane including 15 to 85′% voids that does not collapse after beingbriefly compressed, has remarkably good adhesion on contact with roughsurfaces and remarkably good flexibility and conformability atsub-freezing temperatures.

U.S. Pat. No. 5,134,174 describes biaxially oriented microporouspolypropylene films made using beta-nucleation and specific processingtemperatures. The micro porous films are open-celled with a highporosity of 30-40% with average pore size of 200-800 Angstroms.

U.S. Pat. No. 4,975,469 describes oriented porous polypropylene-basedfilms using beta-nucleating agents. The pores have typical diametersranging from 0.2 to 20 microns and inter-connect with each other and are“open-celled” such that the porous film exhibits a high moisture vaportransmission rate of about 2500-7500 g/m²/day. The beta-crystallineportions are extracted via a sol vent to form a porous film.

Canadian patent application No. CA02551526 describes a biaxiallyoriented white polypropylene film for thermal transfer recordingincluding a polypropylene resin of 30% or higher beta crystal ratio and140-172° C. melting temperature, in which the biaxially oriented whitepolypropylene film has substantially non-nucleated voids. A receivinglayer is provided on one side of the film for thermal transfer recordingin which the receiving layer includes at least one or more kinds ofresin selected from polyolefin, acryl-based resin, polyester-basedresin, and polyurethane based resin.

SUMMARY OF THE INVENTION

This invention addresses the issue of potential chemical hazards used inthermally-reactive coated substrates by making a label film that cancreate a distinct visual contrast via a physical change to the film. Theinvention eliminates the need for unreacted chemicals to createcustomizable labels. By using the micro-voids formed by beta-nucleationof the propylene-based substrate in the top visual layer and theselective application of heat from a thermal print-head to specificportions of this micro-voided top layer (i.e. in the shape of images oralphanumeric characters), the beta-nucleated voids collapse and thusprovide a transparent film in the heated areas. This contrast between anopaque, white microvoided area and a non-voided transparent area allowsthe formation of discrete images and alphanumeric characters as desired.In addition, since the temperature required to collapse the microvoidsis much higher than typical ambient conditions, such images are expectedto be more durable and resistant to fading over time and ambientenvironmental exposure than the current art using thermally-reactivecoatings and inks.

The film of this invention works with a non-reversible physical changein the structure of the film to go from an opaque white or Light-coloredappearance that is due to micro-voids in the film, to selectivelycollapsing or eliminating the voids to provide a transparent, clear filmin the area where heat has been applied. By laminating the film to adarker-colored or contrasting colored substrate- or by coating one sideof the film with a dark-colored or contrasting colored ink orcoating—such contrasting colors will show through the clear ortransparent areas of the inventive film. As this visual change (fromwhite to clear) requires enough heat to change the structure of thefilm, i.e., a softening point of about 148° C., it is not likely thatthe custom information would fade or be converted over a wide area dueto aging or ambient environmental conditions, such as exposure tosunlight or other outdoor weather conditions.

Polypropylene is not known for its resistance to sunlight but a shortexposure of a few days or even up to a year, would not be expected tohave any effect on the film's imagery after thermal printing.Nevertheless, robustness to prolonged exposure, such as is need for anagricultural plant tag, could be provided by modifying the film toincorporate an UV stabilizer or blocker to preserve the polypropyleneand ensure durability in harsher environmental conditions.

A dark background would be necessary to create a good contrast where the“thermal print” is to occur, but the color does not need to berestricted to black or white. The white surface, while easiest to createwith the micro-voids, may also be colored or pigmented, as long as thecolor would consist of a transparent pigment, which would still resultin sufficient contrast against the dark background. Similarly, thecontrasting color need not be black but any suitable color or shadingthat provides enough contrast with the micro-voided film to distinguishthe “printed” information for the naked eye or machine readers.

The background on creating the micro-voided or cavitated film was in thefood packaging industry, so all the components of the film can easily bemade to comply with FDA packaging standards. Specifically, the method tocreate this film utilizes a beta-crystalline nucleating agent forpolypropylene. Polypropylene can exist in several crystalline forms:alpha, beta, gamma, delta, and smectic crystal forms. Of interest inthis invention are the alpha and beta forms. The specific conditions toproduce a polypropylene article rich in beta-crystals is well-known inthe art, typically requiring specific processing conditions and usuallywith a specific beta-nucleating agent. In this invention, it requires ahot casting roll that, in conjunction with the nucleator additive,causes the formation of a less dense beta crystalline form for thecrystal portions of the semicrystalline polypropylene film. When thisresulting film is stretched or oriented in the machine direction, thebeta crystal changes to the “preferred” denser alpha crystal form forpolypropylene. This change in density creates small micro-voids that,with the orientation, are enlarged enough such that the voids impart awhite, opaque appearance to the film. This cavitation is also shown by areduced density in the resulting film. Stretching the sheet eithermonoaxially or biaxially produces opaque, cavitated film with lowereddensity, high strength, and enhanced printability.

One embodiment is a two-layer mono-axially oriented coextruded film(MOPP) including a microvoided main layer A of a propylene-based polymerincluding an impact ethylene-propylene copolymer and an amount ofbeta-crystalline nucleating agent; and a second layer B of apropylene-based polymer including a crystalline isotactic propylenehomopolymer and an amount of pigment such as a carbon block pigment; inwhich the second layer B is contiguously attached to one side of layer Aand is coextruded as a skin layer or sublayer with the main layer A.This second layer B is not required to be microvoided.

Another embodiment is as a two-layer laminate structure in which onelayer A is a microvoided mono-axially oriented extruded film or sheetincluding an impact ethylene-propylene copolymer and an amount ofbeta-crystalline nucleating agent; and a second layer B is a pigmentedsheet or film including a polymeric or paper sheet containing a pigment,(preferably a color that is in contrast with layer A′s color orappearance). Layer B is laminated or adhered contiguously to one side oflayer A by various means well-known in the art, such as adhesivelamination or extrusion lamination processes. Layer B is not required tobe microvoided.

In an another embodiment, it can also be contemplated to coat the secondlayer B onto one side of main layer A (as described in the previousembodiments) by various means well-known in the art such asextrusion-coating or solution coating. For example, it can becontemplated to extrusion-coat a polyethylene-including melt or otherpolymeric coating such as polypropylene or other polymer types (it mayalso be contemplated to use tie-resin materials, layers, primers,discharge-treating, etc., as needed to improve bonding between layers Aand B), pigmented with carbon black (or other color suitably contrastingwith layer A), onto one side of layer A.

In yet another embodiment, it can be contemplated to apply layer B ontoone side of layer A (as described in the previous embodiments) in whichlayer B is comprised of an ink or inks which provide a contrasting colorto layer A. The inks may be solvent-borne or aqueous (or UV orelectron-beam curable ink systems), and may be applied by various meanswell-known in the art such as flexographic plates or rotogravure rolls;in addition, it can be contemplated to use primers or other materials toimprove bonding of the inks to layer A. It can also be contemplated todischarge-treat the side of layer A which is to receive layer B printinginks by various means well-known in the art such as corona, flame, oratmospheric plasma treating systems; as well as using priming materialsin combination with discharge-treatment methods. It may also becontemplated to apply the layer B printing inks as full coverage overthe chosen side of layer A or as a discrete pattern, preferably inalignment with the desired thermal printing pattern applied to layer Aby a thermal head printer. This latter embodiment may be useful as acost-savings method to reduce the amount of ink coverage needed.

All these embodiments may also include additional additives in layer Aand/or B, such as antiblock particles, slip agents, process aids,antistatic agents, defoamers, adhesion promoters, etc., as needed toenhance processability and other film handling properties. Theseadditives may be added in quantities as described later in thespecification so as not to materially affect or interfere with the basicproperties of the film of this invention.

This invention provides a method to impart thermal print images andinformation via the application of heat only, or with ultrasound, andwithout the need of a chemical change of a thermally active compound.Additional advantages of this invention will become readily apparent tothose skilled in the art from the following detailed description,wherein only the preferred embodiments of this invention are shown anddescribed, simply by way of illustration of the best mode contemplatedfor carrying out this invention. As will be realized, this invention iscapable of other and different embodiments, and its details are capableof modifications in various obvious respects, all without departing fromthis invention. Accordingly, the examples and description are to beregarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a film according to an embodiment of theinvention. Shown is a mono-oriented polyolefinic film 100, comprised oftwo layers. Layer 101 is a white or light-colored/pigmented opaquebeta-nucleated and micro-voided polypropylene layer. Layer 102 is ablack or dark-colored/pigmented polypropylene layer upon one side ofLayer 101. The side of 101 opposite to 102 is the thermal print side.

FIG. 2 is a top view of a thermally exposed 2-layer film sheet 100 ofthe invention showing the change in appearance where a heated platen wasapplied to white beta crystalline micro-voided top layer 101, thuschanging its appearance from white to clear in that area, and allowingblack pigmented bottom layer 102 to show through the transparent portionof top layer 101.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment of the invention, the laminate film is a two-layermono-oriented coextruded film including a first layer A of a polyolefinresin layer including a propylene-based polymer and an amount of abeta-nucleating agent or beta-nucleated propylene polymer; and a secondlayer B coextruded with layer A contiguously upon one side of layer A.Layer B is comprised also of a propylene-based polymer and an amount ofcarbon black pigment in propylene-based polymer carrier resin. Ifdesired, one or both sides of the laminate film structure may bedischarge-treated.

The coextruded polyolefin resin layers A and B were uniaxially (ormonoaxially) oriented. It can be contemplated to biaxially orient thelaminate film as well in both the machine (MD) and transverse (TO)directions. The propylene-based polymer can be an isotacticethylene-propylene impact copolymer with an ethylene-propylene rubbercontent of about 10-30 wt % of the polymer wherein the ethylene contentof the rubber is about 10-80 wt % of the rubber. Typically, thecopolymer is an ethylene-propylene copolymer, an ethylene-butenecopolymer, a propylene-butene copolymer, or an ethylene-propylene-butenecopolymer. Preferably, an ethylene-propylene orethylene-propylene-butene copolymer is used. The copolymer may be anelastomer or plastomer. A thermoplastic elastomer can be described asany of a family of polymers or polymer blends (e.g. plastic and rubbermixtures) that resemble elastomers in that they are highly resilient andcan be repeatably stretched and, upon removal of stress, return to closeto its original shape; is melt-processable at an elevated temperature(uncrosslinked); and does not exhibit significant creep properties.Thermoplastic elastomers typically have a density of between 0.860 and0.890 g/cm3 and a molecular weight Mw of 100,000 or greater. Plastomersdiffer from elastomers: a plastomer can be defined as any of a family ofethylene-based copolymers (i.e. ethylene alpha-olefin copolymer) whichhas properties generally intermediate to those of thermoplasticmaterials and elastomeric materials (thus, the term “plastomer”) with adensity of less than about 0.900 g/cm³ (down to about 0.865 g/cm³) at amolecular weight Mw between about 5000 and 50,000, typically about20,000 to 30,000. Plastomers generally have an ethylene crystallinitybetween thermoplastics and ethylene alpha-olefin elastomers and aregenerally of a higher crystallinity than elastomers (which can generallybe considered amorphous). As such, plastomers generally have bettertensile properties than elastomers.

A suitable example of ethylene-propylene impact copolymer for thisinvention is Total Petrochemical's 5571. This resin has a melt flow rateof about 7 g/10 minutes at 230° C., a melting point of about 160-165°C., a Vicat softening point of about 148° C., and a density of about0.905 g/cm³. Another example of ethylene-propylene impact copolymer canbe Total Petrochemical's 4180 with a melt flow rate of about 0.7 g/10minutes at 230° C., a melting point of about 160-165° C., a Vicatsoftening point of about 150° C., and a density of about 0.905 g/cm³.Other suitable ethylene-propylene impact copolymers can be Braskem'sTI-4015-F with an ethylene-propylene rubber of 10-30 wt %, a melt flowrate of 1.6 g/10 minutes at 230° C., melting point of 160-165° C., Vicatsoftening point of 148° C., and a density of about 0.901 g/cm³; andExxonMobil Chemical's PP7033E2 with a melt flow rate of about 8 g/10minutes at 230° C. and a density of about 0.9 g/cm³.

Other suitable propylene-based polymers can be isotactic crystallinepropylene homopolymers and “mini-random” isotactic crystallineethylene-propylene copolymers. “Mini-random” propylene homopolymers arethose class of ethylene-propylene copolymers in which the ethylenecontent is fractional, i.e. less than 1 wt %, typically on the order ofabout 0.2-0.8 wt %, and preferably about 0.5-0.7 wt %. These crystallineisotactic polypropylenes are generally described as having an isotacticcontent of about 90% or greater as measured by C13 NMR. Suitableexamples of crystalline propylene homopolymers for this invention areTotal Petrochemicals 3271 and 3373HA, Phillips CH016 and CR035, andBraskem FF018. These resins also have melt flow rates of about 0.5 to 5g/10 min at 230°, a melting point of about 163-167° C., acrystallization temperature of about 108-126° C., a heat of fusion ofabout 86-110 J/g, a heat of crystallization of about 105-111 J/g, and adensity of about 0.90-0.91. Higher isotactic content propylenehomopolymers (i.e. “high crystalline” homopolymers) may also be used.Suitable examples of these include those made by Total Petrochemicals3270 and 3273 grades, Braskem grade HR020F3, and Phillips 66 CH020XK.These high crystalline polypropylenes typically have an isotacticcontent of 93% or greater as measured by 13C NMR spectra obtained in1,2,4-trichlorobenzene solutions at 130° C. The % percent isotactic canbe obtained by the intensity of the isotactic methyl group at 21.7 ppmversus the total (isotactic and atactic) methyl groups from 22 to 19.4ppm. These resins also have melt flow rates of about 0.5 to 5 g/10 min,a melting point of about 163-167° C., a crystallization temperature ofabout 108-126° C., a heat of fusion of about 86-110 J/g, a heat ofcrystallization of about 105-111 J/g, and a density of about 0.90-0.91.

In the case of using high crystalline propylene homopolymers, it mayalso be contemplated to employ processing aids to help improveorientation, lowering orientation stresses, uneven stretching marks,motor draw amps, etc. Examples of suitable processing aids can be thosebased on hydrocarbon resins of various types. In particular,polydicyclopentadiene hydrocarbon resins are preferred processing aidsas having good clarity, no smoking issues, no odor issues, and goodmiscibility with propylene-based resins. As a processing aid, inclusionof the hydrocarbon resin allows a wider “processing window” in terms ofprocessing temperatures for machine direction (MD) and/or particularly,transverse direction (TD) orientation. A suitable hydrocarbon resin isof the polydicyclopentadiene type available in masterbatch form fromExxonMobil as PA639A, which is a 40% masterbatch of polypropylenecarrier resin and 60% hydrocarbon resin. Suitable amounts of thehydrocarbon masterbatch to use in Layer A and/or Bare concentrations ofup to 10% masterbatch or up to 5% of the active hydrocarbon resincomponent. The pure hydrocarbon resin can also be obtained (i.e. not asa masterbatch) as ExxonMobil PR100A.

Suitable examples of propylene-based random copolymers for thisinvention are: Total Petrochemicals Z9421 ethylene-propylene randomcopolymer elastomer of about 5.0 g/10 min melt flow rate (MFR) at 230°C., melting point of about 120° C., density 0.89 g/cm³, and ethylenecontent of about 7 wt % of the polymer; Total Petrochemicals 8473ethylene-propylene random copolymer of about 4.0 MFR at 230° C. andethylene content of about 4.5 wt % of the polymer; Sumitomo ChemicalSPX78R1 ethylene-propylene-butene random copolymer of about 9.5 g/10 minMFR at 230° C., ethylene content of about 1.5 wt %, and butene contentof about 16 wt % of the polymer; or ExxonMobil Chemical Vistamaxx™ethylene-propylene random copolymer elastomers such as grade 3980 FLwith an MFR of about 8.3 g/10 min at 230° C., Vicat softening point ofabout 80° C., melting point of about 79° C., density of about 0.879g/cm³, and ethylene content of about 8.5 wt %. Other suitablepropylene-based copolymers and elastomers may be contemplated includingbut not Limited to: metallocene-catalyzed thermoplastic elastomers likeExxonMobil's Vistamaxx™ 3000 grade, which is an ethylene-propyleneelastomer of about 11 wt % ethylene content, 8 g/10 min MFR at 230° C.,density of 0.871 g/cm³, T_(g) of −20 to −30° C., and Vicat softeningpoint of 64° C.; or ethylene-propylene alpha-olefin copolymer plastomersof Dow Chemical's Versify™ grades, such as grade 3300, which is anethylene-propylene plastomer of about 12 wt % ethylene content, 8 g/10min MFR at 230° C., density of 0.866 g/cm³, T_(g) of −28° C., and Vicatsoftening point of 29° C.; and Mitsui Chemicals Tafmer™ grades XM7070and XM7080 metallocene-catalyzed propylene-butene random elastomers ofabout 22 and 26 wt % butene content, respectively. They arecharacterized by a melting point of 75° C. and 83° C., respectively; aVicat softening point of 67° C. and 74° C., respectively; a density of0.883-0.885 g/cm³; a T_(g) of about −15° C.; a melt flow rate at 230° C.of 7.0 g/10 minutes; and a molecular weight of 190,000-192,000 g/mol.

Additionally, an amount of inorganic antiblocking agent may beoptionally added up to 5000 ppm to either or both resin Layers A and Bas desired for film-handling purposes, winding, antiblocking properties,and control of coefficient of friction. Preferably 300-5000 ppm, andmore preferably 500-1000 ppm, of antiblock may be added. Suitableantiblock agents comprise those such as inorganic silicas, sodiumcalcium aluminosilicates, crosslinked silicone polymers such aspolymethylsilsesquioxane, and polymethylmethacrylate spheres. Typicaluseful particle sizes of these antiblocks range from 1-12 urn,preferably in the range of 2-6 μm.

Migratory slip agents such as fatty amides and/or silicone oils can alsobe optionally employed in either or both film layers, either with orwithout the inorganic antiblocking additives, to aid further withcontrolling coefficient of friction and web handling issues. Suitabletypes of fatty amides are those such as stearamide or erucamide andsimilar types, in amounts of 100-5000 ppm of the layer. Preferably,erucamide can be used at 500-1000 ppm of the layer. A suitable siliconeoil that can be used is a low molecular weight oil of 350 centistokeswhich blooms to the surface readily at a loading of 400-600 ppm ofeither or both layers.

The beta crystalline phase in polypropylene differs from the alphacrystalline phase as mentioned previously. The alpha phase is the mostcommon crystalline phase and has a melting point typically of about 164°C. whereas the beta phase is less common and has a melting pointtypically of about 150° C. Microvoids can form in the substrate duringorientation when in the solid state, due to the transformation of thebeta crystals into alpha crystals, and this accounts for the whiteopaque appearance of the inventive film's layer A. These microvoids cancollapse upon melting and recooling of the substrate and the whiteopaque appearance can turn transparent and clear; without being bound byany theory, it is this property that gives the unique non-chemicalthermal print opportunities of the inventive film as a thermal printinghead is put in contact with the beta-nucleated and microvoided layer Aand is activated.

Beta nucleating agents are well-known and studied. Truly effective betanucleators are not common, but effective beta nucleators have been foundbased on materials such as: pimelic acid supported on nano-CaCO3; amidesof dicarboxylic acid (e.g.N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide; aryl dicarboxylic acidamide); two-component beta nucleating agents of organic dibasic acids(such as pimelic acid, azelaic acid, o-phthalic acid, terephthalic acid,isophthalic acid) and oxide, hydroxide, or acid salts of a Group IImetals (e.g. magnesium, calcium, barium); gamma-crystalline form ofquinacridone colorant; aluminum salt of 6-quinizarin sulfonic acid;bisodium salt of o-phthalic acid. Beta nucleating agents areconveniently obtained as a commercial masterbatch in a polypropylenecarrier resin; a suitable one for use is Mayzo Corporation's BNX®MPM1112 grade beta nucleant polypropylene masterbatch with melt flowrate of 12 g/10 min at 230° C., dual melting point of 150-155° C. forbeta crystal phase and 162-167° C. for alpha crystal phase (as measuredon a 2 wt % letdown ratio of the masterbatch in propylene homopolymerresin via second heat using a differential scanning calorimeter), andspecific gravity of 0.90 g/cm³. Suitable amounts of this masterbatch touse in layer A of the inventive film is from about 0.5 to 3.0 wt % ofthe layer, preferably about 1.0 to 2.0 wt %, and more preferably, about1.1 wt % to 1.8 wt %.

For the contrasting color layer B of this coextruded two-layer filmembodiment, carbon black pigment may be used (although other contrastingcolors may also be used). Carbon blacks are commonly and widely used aspigments, colorants, and fillers for rubber and plastic products. Carbonblacks are typically produced from the charring of organic materialssuch as wood or bone; or the incomplete combustion of petroleum productsand/or vegetable oils. Carbon black pigments are most conveniently usedand handled in a masterbatch form and a suitable one for the presentinvention can be obtained from Ampacet Corporation as grade 19114 FDABlack carbon black pigment in a polypropylene carrier. This masterbatchhas typical properties of a 4 g/10 min melt flow rate at 230° C.,melting point of 160-165° C., and density of 1.13 g/cm³. Suitableamounts to use in layer B for a suitable dark color is about 1-20 wt %of the layer, and preferably, about 6-9 wt %.

The beta-nucleated resin layer A can be 20 μm to 200 μm in thicknessafter monoaxial orientation, preferably between 30 μm and 150 μm, andmore preferably between 70 μm and 100 μm in thickness. The coextrudedlayer B of this embodiment can be between 2-200 μm in thickness, but anythickness may be chosen that is suitable for the contrast ratio betweenthe clear and dark areas after thermal printing. The main criteria is toensure a thick enough coextruded layer B to reasonably and sufficientlycontain enough pigment to provide a good contrasting color. Preferably,the thickness of both A and B layers combined should be in the range of25 to 200 μm, more preferably, 100 to 200 μm. The ratio of layer A tolayer B thickness can be varied and optimized to meet specific end-useapplications for thermal print substrates and labels.

The surface of layer A opposite layer B can also be surface treated witheither an electrical corona-discharge treatment method, flame treatment,atmospheric plasma, or corona discharge in a controlled atmosphere ofnitrogen, carbon dioxide, or a mixture thereof, with oxygen excluded andits presence minimized. The latter method of corona treatment in acontrolled atmosphere of a mixture of nitrogen and carbon dioxideresults in a treated surface that comprises nitrogen-bearing functionalgroups, preferably at least 0.3 atomic % or more, and more preferably,at least 0.5 atomic % or more.

In the above embodiment of a coextruded two-layer film, the respectivelayers can be coextruded through a multi-layer compositing die such as a2-layer die, and cast onto a chill roll to form a solid film suitablefor further processing. In the case of a single layer film, therespective layer can be extruded through a single-layer die and castonto a chill roll to form a solid film suitable for further processing.Extrusion temperatures are typically set at 235-275° C. with a resultingmelt temperature at the die of about 230-250° C. Preferably, theextrusion profile of beta-nucleated layer A is a “reverse” temperatureprofile in which the feed zones of the extruder are set higher than thefinal zones. In this case, suitable extrusion temperature settings arefrom about 271° C. in the initial feed zone, to about 240° C. in thefinal zone. Filter and melt pipe temperatures were set at about 240° C.;die temperature was about 232° C.

The inventive laminate film was extruded into a sheet form and cast ontoa cooling drum at a speed of 6 to 15 mpm whose surface temperature iscontrolled between 99° C. and 104° C. to solidify the non-orientedlaminate sheet. These higher casting temperature conditions areimportant to form and favor beta crystal formation.

The laminate film was monoaxially oriented in the machine direction (MD)to a certain amount. The amount of monoaxial machine directionorientation should be about 2.5-7 times in the machine direction,preferably 3-7 times, and more preferably 4.0 to 7.0 times. Above a 7:1machine direction orientation ratio, processability issues may resultsuch as film breakage which can affect the product cost and machineefficiency; below a 2.5:1 machine direction orientation ratio,processability issues such as uneven film profile, gauge bands, anduneven stretch marks can occur which also can result in higher productcosts and lower machine efficiencies. Once oriented at the appropriatestretch ratio, the laminate film's layer A appears white and opaque dueto the formation of micro voids around the beta crystal sites. It shouldbe noted that that the microvoids of the inventive film wereclosed-cell, and not open-cell, and thus, did not result in continuouspores which made the microvoided film porous. The density of themicrovoided beta-crystalline layer A ranged from about 0.77 to 0.80g/cm³.

MD orientation temperatures were typically set at about: 113° C. forpreheat rolls; 93° C. for stretching; and 126° C. for annealing.Annealing or heat-setting in the final sections of the MD orientationunit was used to help reduce internal stresses within the laminate filmand minimize heat shrinkage and maintain a dimensionally stablemono-axially oriented film.

The uniaxially oriented sheet was then optionally passed through adischarge-treatment process on one side of the film (i.e. the side oflayer A opposite layer B) such as an electrical corona discharge treaterat a watt density of about 2.4 watt/ft². The one-side treated film wasthen wound into roll form. The finished article appeared as a film withone side white (layer A) and the opposite side dark (layer B).

Further embodiments may be contemplated as well. In one embodiment, itmay be contemplated to produce at least a single layer A only of themono-axially oriented microvoided film comprised of a propylene-basedpolymer and the betacrystalline nucleating agent or masterbatch, andlaminating film A to a second, standalone, dark or contrasting coloredor pigmented film or substrate C by means well-known in the art with anadhesive. This adhesive lamination may be accomplished by using anynumber of aqueous or solvent-borne adhesives (e.g. 2-part urethane) viawell-known solution coating methods including, but not limited to,gravure or rod coating methods; solventless-lamination methodsincluding, but not limited to, extrusion lamination using moltensolventless adhesives such as low density polyethylene, or hot meltsystems; solventless adhesive systems such as UV or electron beamcurable adhesives using application methods including, but not limitedto, gravure or rod coating methods. Such adhesives may be applied to oneside of the colored film C or to one side of the opaque micro-voidedbeta-crystalline propylene-based polymer film of layer A as desired forthe lamination process.

The colored film or substrate C of the above embodiment may be opaque ortranslucent, but should be a separate film from the film made of layerA. It can be produced in a separate film-forming process as the film oflayer A. For example, one could extrude (or coextrude a multi-layerfilm) film C as including a propylene-based film and carbon black of theformulation described previously for layer B. Film C could range inthickness including, but not limited to, from 1 μm to 100 μm as desired.The contrasting colored film C may be comprised (but not limited to) of:paper; paperboard; cellulosic films; metal foils and films; polymericfilm or films including polypropylene, polyethylene, polyethyleneterephthalate, polyester, nylon, polylactic acid, polystyrene, otherpolymers; metallized substrates (e.g. paper or polymeric films); orcombinations of substrates.

In another embodiment, it may be contemplated to apply a contrastingcolored ink or pigmented coating to one side of the film of layer A. Forprinting an ink or applying a pigmented coating to one side of the layerA film, it may be desirable to discharge-treat the side of interest ofthe film, to help promote wet-out and adhesion of the ink or coating.Primers or other adhesion promotes may be used as well for this purpose.In the case of printing, an ink—e.g. a black ink or other contrastingcolored ink could be applied to one side of the film of layer A by meanswell-known in the art such as gravure roll or flexographic plates. Theink may be water-based, solvent-based, or solventless type that is curedby UV or electron beam. A contrasting colored coating may also beapplied to one side of the film including layer A. For example, a carbonblack containing polymeric coating may be extrusion-coated on one sideof the film including layer A; alternatively, an aqueous orsolvent-based coating may be applied to one side of the film includinglayer A via gravure or rod coating or other means well-known in the art;further, both an ink and a coating may be applied together to one sideof the film including layer A, in which the ink is applied directly tothe film and the coating applied on top of the ink. The latter may beadvantageous in that the coating—being thicker than the ink—may addadditional opacity and contrast to support the ink pigment or color, andneed not be the same color as the ink. The coating may be white opaque(same color as the film including layer A) for example as long as theink between the coating and layer A film is of a contrasting color tothe layer A film. In addition, the coating may also be useful to helpprotect the ink layer from scuffing or wear (and may also be transparentor unpigmented if used for this purpose). It could also be contemplatedto coat or deposit a metallic layer (e.g. aluminum metallization) toprovide a suitable contrast.

In the above embodiments, it could further be contemplated that the filmincluding layer A could also be a coextruded multi-layer film, forexample, at least a 2-layer coextruded film, in which both layers A andsecond coextruded layer B are both comprised of the propylene-basedpolymer and the beta-crystalline nucleating agent. This multi-layer filmcould then be laminated to the separate contrasting colored layer C orprinted on one side with a contrasting colored pigment or ink.

In a typical thermal printing application, the thermal print head (orheads) is used to transfer ink or dye from the ink or dye donor elements(e.g. thermal transfer ink-containing film or ribbon) to a receiving orrecording element (e.g. print receiving substrate). Alternatively, thethermal print head may contact a substrate containing a thermallysensitive ink or dye that changes color or becomes visible uponapplication of heat from the thermal print head. Other known sources fortransferring or activating thermal inks or dyes, such as lasers, may beused. A thermal ink or dye transfer assemblage may include 1) an ink ordye-donor element; 2) an ink or dye receiving or recording element, theink or dye-receiving element being in a superposed relationship with theink or dye-donor element such that the ink or dye layer of the donorelement may be in contact with the ink or dye-receiving layer of thereceiving element.

In the case of the present invention, as an example using the embodimentincluding a coextruded 2-layer film of a beta-crystalline nucleatedwhite opaque propylene-based polymer layer A and a contrasting coloredpropylene-based layer B, it is contemplated that the laminate filmstructure of layers A and B are fed into the thermal print head assemblysuch that the side of layer A including the beta-nucleated andmicrovoided layer is subjected to heat treatment from the thermal printhead. Upon activation of the thermal print head heating elements,sufficient thermal energy is transferred into layer A to partially meltthe layer in the region of thermal contact, thus transforming themicro-voided beta crystalline regions into alpha crystals. This rendersthe appearance of the thermally-contacted areas from white (or opaque)to clear (or transparent). Thus, the physical contrast between theopaque portions and the transparent portions can be visible by eye(looking through the film from the layer A side wherein the contrastinglayer B underneath shows through the clear portions of Layer A) andinformation conveyed via thermal printing head onto the inventivereceiving substrate without the use of chemical inks or dyes.

In yet another embodiment, it could be contemplated that the nonchemicalthermal print film of the invention could include only a single layer Aincluding a propylene-based polymer and an amount of beta-nucleatingagent. After orientation, the essentially mono-Layer film has amicro-voided white opaque appearance and, after passing through thethermal print head, the thermally “printed” areas of layer A turn fromwhite to clear, thus providing enough contrast to discern informationsuch as alphanumeric lettering and/or barcodes or other information.

It can also be contemplated that in addition to thermal print heads,ultrasonic energy may also be sufficient to collapse thebeta-crystalline micro-voids, thus converting the white opaque regionsinto transparent regions where the ultra-sonic energy is directed.

This invention will be better understood with reference to the followingexamples, which are intended to illustrate specific embodiments withinthe overall scope of the invention.

Example 1

A two-layer mono-axially oriented film (MOPP) was made using a monoaxialorientation process with two distinct coextruded layers. The two layerscomprised a black pigmented layer B and a white opaque beta-crystallinenucleated layer A and the laminate film appeared dark colored on oneside (layer A) and light colored on the opposite side (layer A) afterorientation. Layer A was composed of about 98.2 wt % of an impactpropylene copolymer Braskem TI4015F and about 1.8 wt % of abetacrystalline nucleating masterbatch Mayzo MPM1112. Layer B wascomposed of about 94 wt % Braskem TI4015F and about 6 wt % of a carbonblack pigment masterbatch Ampacet 191114. The coextruded film substratewas made via co-extrusion through a die, cast on a temperaturecontrolled drum at 104° C., oriented in the machine direction through aseries of heated and differentially sped rolls at various orientationdraw ratios (MDX) of about 4:1. Once oriented the clear layer B appearedwhitish and opaque due to the formation of microvoids from thebeta-nucleation. Thus, the finished article appeared as a film with ablack side and a white side (FIG. 1). The film was heat-set or annealedin the final zones of the MD orientation section to reduce internalstresses and minimize heat shrinkage of the film and maintain adimensionally stable mono-axially oriented film. After orientation, thefinished thickness of the 2-layer laminate coextruded film was nominal150 μm or 6000 G. The thickness of the beta-nucleated layer A was about135 μm; the thickness of the carbon black pigmented layer B was about 15μm.

Example 2

Example 1 was substantially repeated except that the amount of MayzoMPM1112 beta-nucleating masterbatch was about 2.2 wt % and about 97.8 wt% Braskem TI4015F in layer A.

Example 3

Example 1 was substantially repeated except that the amount of MayzoMPM1112 beta-nucleating masterbatch was about 1.2 wt % and about 98.8 wt% Braskem TI4015F in layer A; the amount of carbon black Ampacet 191114was about 9 wt % and about 91 wt % Braskem TI4015F in layer B; and theoverall thickness was about 100 μm with the A layer about 90 μm and theB layer about 10 μm.

Example 4

A mono-axially oriented film was made as in the above Example 1.However, in this case, both layers A and B were comprised of thebeta-nucleated impact copolymer of about 98.2 wt % ofBraskem TI4015F andabout 1.8 wt % of Mayzo MPM1112, effectively producing a single layerfilm of the same composition throughout. A black pigmented film wasobtained commercially from an outside vendor and adhesively laminatedwith a 2-part urethane adhesive to the beta-nucleated white opaque film.

In the Examples above, thermal printability was tested using alaboratory heat sealing device Sentinel model 12 ASL wherein thebeta-crystalline micro-voided white opaque side of the film (layer A)was exposed to the heated platen (the other sealing platen or jaw wasunheated) of the heat sealer at 320° F. (160° C.) at a dwell time of 0.8and 20 psi pressure, whereupon that portion of the white surface oflayer A subjected to heat, turned transparent and lost its opacity,allowing the darker layer B beneath to show through (FIG. 2). A secondtest was also done at the same temperature and pressure, except that thedwell time was increased to 2.0 seconds (FIG. 2).

Basic properties of the Examples are shown in Table 1.

Light Glass Gloss Black- Density Exam- Transm A- B- Color A -layer nessA-layer ple % side side L* a* b* B-layer g/in³ Ex 1 12.5 17 18 95.84−0.35 0.17 1.39 0.79 Ex 2 14.6 19 16 94.36 −0.29 0.34 1.34 0.78 Ex 320.3 17 16 95.76 −0.32 0.23 1.46 0.78Test Methods

The various properties in the above examples were measured by thefollowing methods:

Thermal Printability: Evaluated using a Sentinel sealer model 12 ASL atabout 20 psi, 0.5-2.0 second dwell time, with heated flat upper seal jawTeflon coated, and unheated lower seal jaw, rubber with glass clothcovered. The film sample is placed 24 between the sealer jaws at thedesired seal temperature(s) in the Sentinel sealer (e.g. 320° F. or 160°C.). Temperatures may be increased or decreased at desired intervals,e.g. 10° F. increments for further evaluation of determining the clarityof the thermal “printability”.

Light Transmission of the film was measured by measuring a single sheetof film using a light transmission meter like a BYK Gardner model“Haze-Gard Plus®” substantially in accordance with ASTM D 1003.

Gloss of the film was measured by measuring the desired side of a singlesheet of film via a surface reflectivity gloss meter (BYK GardnerMicro-Gloss) substantially in accordance with ASTM 02457 at a 60° angle.

Wetting tension of the sur faces of interest was measured substantiallyin accordance with ASTM D2578-67.

Lightness L*a*b* was measured using a spectrodensitometer such as X-Ritemodel 528.

Blackness was measured using an optical densitometer such as TobiasAssociates model TBX transmission densitometer.

Density of the film was calculated by taking a stack of 10 sheets(letter paper size e.g. 8.5 inches by 11 inches) of film and cuttingthem via a die of area 33.69 cm² and weighing the cut sheets on ananalytical scale. The 10 sheets are also measured for thickness using aflat-head micrometer to get an average thickness of the film. Themeasured weight and thickness is then used in a calculation to obtaindensity:

$\frac{{Weight}\mspace{11mu}(g)}{{Thickness}\mspace{11mu}({cm}) \times {area}\mspace{11mu}\left( {cm}^{2} \right)} = {{Density}\mspace{14mu}\left( {g\text{/}{cm}^{3}} \right)}$

Film yield is calculated using film density and thickness by thefollowing formula:

$\frac{453.59}{{Density}\mspace{14mu}\left( {g\text{/}{cm}^{3}} \right) \times (2.54)^{2} \times {thickness}\mspace{14mu}({inches})} = {{Yield}\mspace{14mu}\left( {{in}^{2}\text{/}{lb}} \right)}$

Tensile properties such as Young's modulus, ultimate strength, andelongation are measured substantially in accordance with ASTM 0882.

This application discloses several numerical ranges in the text andfigures. The numerical ranges disclosed inherently support any range orvalue within the disclosed numerical ranges even though a precise rangelimitation is not stated verbatim in the specification because thisinvention can be practiced throughout the disclosed numerical ranges.

The above description is presented to enable a person skilled in the artto make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe preferred embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, this invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein. Finally,the entire disclosure of the patents and publications referred in thisapplication are hereby incorporated herein by reference.

What is claimed is:
 1. A multilayer film comprising: a first layercomprising an image-side layer comprising a cavitating agent, a secondlayer comprising a polypropylene-containing layer that is clear orcomprises a pigment, a colorant, an opacifying agent or combinationsthereof; and wherein the multilayer film forms an image without using anink or a dye.
 2. The multilayer film of claim 1, wherein the colorant,the pigment and the opacifying agent are substantially absent from theimage-side layer.
 3. The multilayer film of claim 1, wherein theimage-side layer comprises voids.
 4. The multilayer film of claim 3,therein the image-side layer is opaque due to the presence of the voids.5. The multilayer film of claim 4, wherein the image-side layer changesfrom being opaque to substantially transparent in a region of themultilayer film wherein the voids collapse.
 6. The multilayer film ofclaim 4, wherein the image-side layer changes from opaque tosubstantially transparent in a region of the multilayer film that isheated.
 7. The multilayer film of claim 4, wherein the image is producedin a region of the multilayer film upon collapse of the voids within arange of from 0.1% to 15% of an original thickness of the image-sidelayer.
 8. The multilayer film of claim 1, wherein the image-side layercomprises a material having a density in the range of 0.9 g/cm³ to 1.13g/cm³, a DSC melting point in the range of 40° C. to 165° C., and a meltflow rate in the range of 1.6 dg/min to 4 dg/min.
 9. The multilayer filmof claim 1, wherein the film is treated with a device that impartsselective physical or thermal treatment to the image-side of themulti-layer film to impart the image.
 10. The multilayer film of claim1, wherein the multilayer film is biaxially oriented.
 11. The multilayerfilm of claim 1, wherein the first layer comprises an opaquebeta-nucleated polypropylene-based polymer; and the second layercomprises a dark pigment.
 12. The multilayer film of claim 11, whereinthe dark pigment of the second layer pigment has a color contrastingwith the color of the first layer.
 13. The multilayer film of claim 11,wherein the first layer comprises a propylene-based polymer and anamount of a beta-nucleating agent or beta-nucleated propylene polymer.14. The multilayer film of claim 13, wherein the second layer comprisesa propylene-based polymer and an amount of carbon black pigment inpropylene-based polymer carrier resin.
 15. The multilayer film of claim11, wherein the first layer comprises micro-voids.
 16. The multilayerfilm of claim 11, wherein the first layer comprises a beta nucleatingagent selected from the group consisting of pimelic acid supported onnano-CaCO₃; amides of dicarboxylic acid includingN,N′-dicyclohexylnaphthalene-2,6-dicarboxamide and aryl dicarboxylicacid amide, two-component beta nucleating agents of organic dibasicacids selected from the group consisting of pimelic acid, azelaic acid,o-phthalic acid, terephthalic acid and isophthalic acid; oxide,hydroxide, or acid salts of Group II metals; gamma-crystalline forms ofquinacridone colorants; aluminum salt of 6-quinizarin sulfonic acid;bisodium salt of o-phthalic acid; and combinations thereof.
 17. Themultilayer film of claim 11, wherein the first layer can be madetransparent upon the application of heat by collapsing the voids in saidfirst layer.
 18. The multilayer film of claim 11, wherein the firstlayer can be made transparent upon the application of ultrasonic energy.19. The multilayer film of claim 11, wherein the first layer comprises atransparent dye.
 20. The multilayer film of claim 11, wherein the darkpigment of the second layer comprises a carbon black.
 21. The multilayerfilm of claim 19, wherein the second layer comprises a transparent dyethat is of a different hue or color than that of the first layer.